In a wireless network, a wireless device may communicate with one or more radio network nodes to send and/or receive information, such as voice traffic, data traffic, control signals, and so on. In some cases, the wireless device may have a connection with multiple radio network nodes of different link quality. A problem may arise where important control information is to be transmitted to the wireless device, but the link quality with a particular radio network node is weak. For example, in a Wideband Code Division Multiple Access (WCDMA) system, a wireless device in soft handover (SHO) is essentially power-controlled by the best uplink (UL) cell. If the best UL is a non-serving cell, it may be difficult to ensure that important control information is reliably received at the serving cell. The problem of weak communication links becomes particularly pronounced when the imbalance between the best UL and downlink (DL) becomes large, such as for heterogeneous networks or multi-flow operation.
Deployment of low-power nodes (LPNs) is seen as a powerful tool to meet the ever-increasing demand for mobile broadband services. A LPN may correspond, for example, to a remote radio unit (RRU), pico, or micro base station. Deployment of LPNs may allow expansion of network capacity in a cost-efficient way. A network consisting of traditional macro NodeBs and LPNs is referred to as a heterogeneous network. Heterogeneous network deployment may be particularly useful in situations where there are coverage holes, as well as for capacity enhancement for localized traffic hotspots.
FIG. 1 is a block diagram illustrating an embodiment of a network 100. Network 100 includes one or more wireless devices 110, radio network nodes 115, radio network controller 120, and core network node 130. Network 100 may be any suitable type of network. For example, network 100 may be a heterogeneous network of the kind described above, and network nodes 115 may be a mixture of macro nodes and LPNs. Wireless device 110 may communicate with a radio network node 115 over a wireless interface. For example, wireless device 110 may transmit wireless signals to radio network node 115 and/or receive wireless signals from radio network node 115. The wireless signals may contain voice traffic, data traffic, control signals, and/or any other suitable information.
Radio network node 115 may interface with radio network controller 120. Radio network controller 120 may control radio network node 115 and may provide certain radio resource management functions, mobility management functions, and/or other suitable functions. Radio network controller 120 may interface with core network node 130. In certain embodiments, radio network controller 120 may interface with core network node 130 via an interconnecting network. The interconnecting network may refer to any interconnecting system capable of transmitting audio, video, signals, data, messages, or any combination of the preceding. The interconnecting network may include all or a portion of a public switched telephone network (PSTN), a public or private data network, a local area network (LAN), a metropolitan area network (MAN), a wide area network (WAN), a local, regional, or global communication or computer network such as the Internet, a wireline or wireless network, an enterprise intranet, or any other suitable communication link, including combinations thereof.
In some embodiments, core network node 130 may manage the establishment of communication sessions and various other functionality for wireless device 110. Wireless device 110 may exchange certain signals with core network node 130 using the non-access stratum layer. In non-access stratum signaling, signals between wireless device 110 and core network node 130 may be transparently passed through the radio access network. Example embodiments of wireless device 110, radio network node 115, and a network node (such as radio network controller 120 or core network node 130) are described with respect to FIGS. 8, 9, and 10, respectively.
Wireless device 110 may communicate with multiple radio network nodes 115. The communication links between wireless device 110 and radio network nodes 115 may be of differing quality. Where important control information is to be transmitted to wireless device 110, but the link quality with a particular radio access node is weak, certain techniques may be used to ensure receipt of the control information.
Soft handover (SHO), also referred to as macro diversity, and fast closed-loop power control are essential features of WCDMA and High Speed Packet Access (HSPA). FIG. 2 illustrates a traditional HSPA deployment scenario with two radio network nodes 115A and 115B having similar transmit power levels, in accordance with certain embodiments. For example, network nodes 115A and 115B may both be macro nodes with similar transmit power levels. Ideally, UE 110A moving from serving cell 115A towards non-serving cell 115B would enter the SHO region at point A 204. At point B 206, a serving cell change would occur. During a serving cell change, the non-serving cell becomes the serving cell and vice versa. For example, during a serving cell change macro node 115A, the current serving cell, would become the non-serving cell, and the current non-serving cell 115B would become the serving cell. At point C 208, UE 110A would leave the SHO region.
A radio network controller, such as radio network controller 120 described above in relation to FIG. 1, is in control of reconfigurations. This may imply rather long delays for performing a cell change. During SHO, UE 110A is power-controlled by the best uplink cell. In the scenario illustrated in FIG. 2, network nodes 115A and 115B have roughly the same transmit power, so the optimal DL and UL cell borders will coincide, i.e., the path loss from UE 110A to network nodes 115A and 115B will be equal at point B 206. Hence, in an ideal setting, and from a static (long-term fading such as shadowing) point of view, the serving cell 115A would always have the best uplink. In practice, however, due to imperfections (e.g., reconfiguration delays) and fast fading, UE 110A might be power controlled by non-serving cell 115B during SHO. In such a case, problems may arise due to the weaker link between serving cell 115A and UE 110A. For example, receiving essential control channel information, such as hybrid automatic repeat request (HARQ) positive acknowledgement/negative acknowledgement (ACK/NACK) feedback for HSDPA and scheduling information for Enhanced Uplink (EUL), in serving cell 115A may be problematic. Furthermore, downlink transmit power control (TPC) commands need to be received in serving cell 115A in order for serving cell 115A to set the transmit power level of the fractional dedicated physical channel (F-DPCH), which carries the uplink TPC commands.
FIG. 3 illustrates a HSPA deployment scenario with two radio network nodes 115A, 115B having different transmit power levels, in accordance with certain embodiments. In FIG. 3, radio network node 115A is a macro node, and radio network node 115B is a LPN. Since macro node 115A and LPN 115B have different transmit power levels, the UL and downlink (DL) cell borders may not necessarily coincide. For example, wireless device 110A has a smaller path loss to LPN 115B, while the strongest received power is from macro node 115A. In such a scenario, the UL is better served by LPN 115B, while the DL is provided by serving macro node 115A.
In FIG. 3, the region between the equal path loss border and equal downlink received power (e.g., common pilot channel (CPICH) receive power) border may be referred to as an imbalance region. In the imbalance region, some fundamental problems may be encountered. For example, wireless device 110A in position A 302 would have macro node 115A as the serving cell, but be power controlled towards LPN 115B. Due to the UL-DL imbalance, the UL towards serving macro node 115A may be very weak. In such circumstances, important control information might not be reliably decoded in serving cell 115A.
This problem may be addressed to some extent by utilizing available RNC based cell selection offset parameters. By tuning the Cell Individual Offset (CIO) parameter, the handover border can be shifted towards the optimal UL border. Similarly, the IN_RANGE and OUT_RANGE parameters may be adjusted in order to extend the SHO region.
FIG. 4 illustrates SHO operation for HSPA in a heterogeneous deployment with range extension, in accordance with certain embodiments. Like FIG. 3, FIG. 4 includes two radio network nodes 115A and 115B having different transmit power levels. More particularly, radio network node 115A is a macro node, and radio network node 115B is a LPN. FIG. 4 illustrates the effect of adjustments to the CIO parameter described above. While the adjustments to the CIO parameter may be beneficial from a system performance point of view, in certain heterogeneous networks the power difference between macro node 115A and LPN 115B may be more than 10 dB. In practice, it is unlikely the CIO parameter will be set to more than 6 dB due to considerations such as DL signaling cost in terms of radio resource consumption. As a result, the imbalance region may not be eliminated by means of CIO setting.
Possible solutions to the above described problems may include increasing the gain factors by means of RRC signalling, utilizing repetition or relying on HARQ. Note, however, that possible imbalances between UL and DL in a macro only network are mainly caused by fast fading in a traditional deployment, whereas for other scenarios, such as heterogeneous networks, other factors make the imbalance more pronounced. Thus, the possible solutions mentioned above may be less effective in a heterogeneous network.
During RAN#56 in September 2012, a study item (SI) was initiated on UMTS Heterogeneous Networks. During the SI, many solutions were proposed to address the problem of scheduling information and HS-DPCCH reception in the serving cell for UEs in the imbalance region having the macro as the serving cell (region B described above in FIG. 4). One proposed solution is to provide a new secondary pilot channel (S-DPCCH) in the uplink.